Electronic structure of CaMn_1-xNb_xO₃ (x=0.02, 0.04, 0.06 and 0.08) perovskites. Self-organization of terminal layers

Electronic structure of niobium doped CaMn_1-xNb_xO₃ (x = 0.02, 0.04, 0.06 and 0.08) powder and fresh chip manganites was studied by x-ray absorption near edge spectroscopy (XANES) at Mn K-edge, and by x-ray photoelectron spectroscopy (XPS) of Mn 3s -, Mn 2p -, O 1s -, Ca 2p- and Nb 3d-core levels. These XPS spectra for all the studied perovskites are very similar. The O 1s and Ca 2p spectra for powders differ from those for fresh chips because of the quick oxidation and carbonization of powder surface. The Mn 3s, Mn 2p and Nb 3d spectra for the powders and fresh chips are almost the same. According to Mn 3s spectra the formal valence of surface manganese ions is 3.82 for all studied oxides indicating the presence of oxygen vacancies in the samples. Niobium is in pentavalent state as evidenced by XPS measurements, but the negative chemical shift of the Nb 3d_5/2 core level with respect to Nb₂O₅ oxide is observed. High similarity of the XPS spectra of the samples with different Nb doping is probably a result of the self-organization of terminal layers.